1. Field of the Invention:
The present invention relates to polyurethane resin (PUR) powder paints based on polymers containing hydroxyl groups and polyisocyanates containing urea groups as hardeners for matt coatings.
2. Discussion of the Background:
For many coatings, a high gloss is not desired. The reason for this is generally of a practical nature. Glossy surfaces require a higher degree of cleaning than matt surfaces; furthermore, it may be necessary for safety reasons to avoid strongly reflecting surfaces.
Matt surfaces can be produced by admixing smaller or larger amounts of fillers such as chalk, finely divided silicon dioxide, or barium sulfate with the powder paint, depending on the desired degree of gloss. Of course, these additives affect the mechanical film properties negatively. The resulting paint films are also frequently porous.
The addition of organic polymeric additives such as polypropylene wax or cellulose derivatives likewise leads to matting but in this case the matting effect is not always reproducible.
Additionally, matting can also be achieved by dry mixing of various powders, optionally of varying reactivity. The involved and tedious preparation as well as the lack of reproducibility of the desired gloss are considered to be drawbacks (for example, see DE-OS 21 47 653 and DE-OS 22 47 779).
The powder paint system for mat coatings described in Japanese Patent Application Disclosure 79/36339 consists of:
(a) 95 to 20% of a PUR system consisting of a polyester polyol with a softening point of 65.degree. to 130.degree. C. and a blocked polyisocyanate, and
(b) 5 to 80% of an epoxyacrylic system consisting of an acrylate resin containing glycidyl ether groups that is cured with a dicarboxylic acid.
This powder paint system suffers from the same drawbacks as mentioned in the case of matting by mixing powders, and, furthermore, when using powders of varying reactivity, internal stresses of the different binder systems cured under different conditions lead to a reduction of paint film properties.
A process is described in DE-PS 23 24 696 for preparing coatings with matt surfaces, in which the salts of cyclic amidines with certain polycarboxylic acids is used to harden epoxy resins.
For powder paints that contain epoxy resins as binder components and are only used indoors, the previous procedures for matting by addition of fillers and/or by incompatibility of the binder components used and/or by dry mixing of various powders of optionally differing reactivity can be circumvented with the matting hardeners proposed in DE-PS 23 24 696.
However, the epoxy resin powder paints described are not suitable for preparing weather-resistant and lightfast coatings. For this application, binders based on saturated polyesters containing hydroxyl groups and/or acrylates and polyisocyanates are used, for example 3-isocyanatomethyl-3,5,5-trimethylcyclohexylisocyanate, also called isophoronediisocyanate (IPDI).
Such weather-resistant powder paints are described in DE-OS 32 32 463. In addition to epsilon caprolactam blocked isocyanate groups they also contain carboxyl groups in a certain ratio, which gives rise to the disadvantage that, in addition to polyesters containing hydroxyl groups, a third binder component, namely polyepoxides, must be used.
Finally, polyurethane powder paints for matt coatings that are based on three binder components are also described in DE-OS 33 28 129. In this case, pyromellitic dianhydride is used in addition to the polyester containing hydroxyl groups and the blocked polyisocyanate.
These procedures also suffer from the disadvantage of not providing a reproducible matting effect.
With these weather resistant polyurethane powder paints, known procedures have had to be used to make matt powders while accepting the substantial drawbacks, i.e.
(a) the drawbacks mentioned in matting with fillers,
(b) the drawbacks mentioned when matting from incompatibility of the binder components used,
(c) the mentioned drawbacks when matting by mixing different powders,
(d) the mentioned drawbacks when matting by using three binder components.
Thus, there is a need for powder paints for matt coatings that do not suffer from the above-mentioned drawbacks.
There is a further need for a process to produce powder paints for matt coatings that do not suffer from the above-mentioned drawbacks.